1. Field of the Invention
The present invention relates to a laminated gas sensor element and a gas sensor incorporating the laminated gas sensor element.
2. Description of the Related Art
A laminated gas sensor element is known as an element of a gas sensor for measuring a concentration or detecting a specific gas component contained in the exhaust gas of an internal combustion engine, etc. Since a solid electrolyte such as zirconia used in this laminated gas sensor element becomes active at a high temperature of 300° C. or more, the solid electrolyte is normally used in a state in which it is heated by a heater laminated to the solid electrolyte. As a result, when oil drops or water drops in a measuring object gas adhere to the gas sensor element, cracking sometimes occurs due to thermal shock. A technique for protecting a detection part of the gas sensor element exposed to the measuring object gas by means of a porous protective layer is known in order to solve this problem. For example, a technique for preventing the occurrence of cracking by thickening a protective layer in the corner of a gas sensor element is known (Patent Reference 1). Also, a technique for suppressing an increase in volume and suppressing thermal shock by making a horizontal width dimension of the top of a gas sensor element having a detection part smaller than that of the other portion and coating a surface along a lamination direction with a protective layer is known (Patent Reference 2).
[Patent Reference 1] JP-A-2003-322632
[Patent Reference 2] JP-A-2006-343297
3. Problems Solved by the Invention
However, the above described techniques have a few disadvantages. That is, the inventor of the present application found that cracking tends to occur at a boundary part between layers constituting a gas sensor element. Concretely, when a protective layer is not formed at the boundary part and the boundary part is exposed or a protective layer with a sufficient thickness is not disposed at the boundary part, oil drops or water drops in a measuring object gas adhere to the boundary part. As a result, thermal shock is imparted due to a difference in thermal shrinkage between mutual layers and a crack sometimes occurs in the layer. On the other hand, the cracking described above can be suppressed by thickening of a protective layer. However, this results in an increase in volume with an increase in the thickness of the protective layer. Consequently, a long activation time is required to heat the sensor element to a predetermined (activation) temperature at which the solid electrolyte is activated. Namely, the longer activation temperature becomes a factor in inhibiting speedy startup of a gas sensor.